Capsaicin Receptor TRPV1 Research Articles

Cutaneous Application of Capsaicin Cream Reduces Clinical Signs of Experimental Colitis and Repairs Intestinal Barrier Integrity by Modulating the Gut Microbiota and Tight Junction Proteins.

Capsaicin, a pungent compound in chili peppers, is described as having potent anti-inflammatory, antioxidant, and antimicrobial properties. It is also described as a potential modulator of the immune system and intestinal microbiota. Oral or rectal administration of capsaicin has been studied to treat or prevent colitis. However, those vias are often not well accepted due to the burning sensation that capsaicin can cause. Our objective was to evaluate whether the application of capsaicin skin creams (0.075%) would be effective in improving inflammation and epithelial barrier function as well as the composition of the gut microbiota in a model of mild colitis induced by dextran sulfate sodium (1.5%). The results showed that the cutaneous application of capsaicin reversed weight loss and decreased colon shortening and diarrhea, all typical signs of colitis. There was also an improvement in the intestinal epithelial barrier, preserving proteins from tight junctions. We also evaluated the biodistribution of 99mtechnetium-radiolabeled capsaicin (99mTc-CAPS) applied to the back skin of the animals. We found significant concentrations of 99 mTc-Cap in the colon and small intestine after 2 and 4 h of administration. In addition, there was an increased expression of capsaicin receptor TRPV1 in the colon. Moreover, animals with colitis receiving cutaneous capsaicin presented a better short-chain fatty acid profile and increased levels of SIgA, suggesting increased microbiota diversity. In conclusion, our work opens avenues for further studies to better understand capsaicin's potential benefits and mechanisms in addressing colitis through cutaneous application.

How much does TRPV1 deviate from an ideal MWC-type protein?

Many ion channels are known to behave as an allosteric protein, coupling environmental stimuli captured by specialized sensing domains to the opening of a central pore. The classic Monod-Wyman-Changeux (MWC) model, originally proposed to describe binding of gas molecules to hemoglobin, has been widely used as a framework for analyzing ion channel gating. Here, we address the issue of how accurately the MWC model predicts activation of the capsaicin receptor TRPV1 by vanilloids. Taking advantage of a concatemeric design that makes it possible to lock TRPV1 in states with zero to four bound vanilloid molecules, we showed quantitatively that the overall gating behavior is satisfactorily predicted by the MWC model. There is, however, a small yet detectable subunit position effect: ligand binding to two kitty-corner subunits is 0.3–0.4 kcal/mol more effective in inducing opening than binding to two neighbor subunits. This difference—less than 10% of the overall energetic contribution from ligand binding—might be due to the restriction on subunit arrangement imposed by the planar membrane; if this is the case, then the position effect is not expected in hemoglobin, in which each subunit is related equivalently to all the other subunits.

Targeting TRPV1 for Cancer Pain Relief: Can It Work?

Chronic intractable pain affects a large proportion of cancer patients, especially those with metastatic bone disease. Blocking sensory afferents for cancer pain relief represents an attractive alternative to opioids and other drugs acting in the CNS in that sensory nerve blockers are not addictive and do not affect the mental state of the patient. A distinct subpopulation of sensory afferents expresses the capsaicin receptor TRPV1. Intrathecal resiniferatoxin, an ultrapotent capsaicin analog, ablates TRPV1-expressing nerve endings exposed to the cerebrospinal fluid, resulting in permanent analgesia in women with cervical cancer metastasis to the pelvic bone. High-dose capsaicin patches are effective pain killers in patients with chemotherapy-induced peripheral neuropathic pain. However, large gaps remain in our knowledge since the mechanisms by which cancer activates TRPV1 are essentially unknown. Most important, it is not clear whether or not sensory denervation mediated by TRPV1 agonists affects cancer progression. In a murine model of breast cancer, capsaicin desensitization was reported to accelerate progression. By contrast, desensitization mediated by resiniferatoxin was found to block melanoma growth. These observations imply that TRPV1 blockade for pain relief may be indicated for some cancers and contraindicated for others. In this review, we explore the current state of this field and compare the analgesic potential of TRPV1 antagonism and sensory afferent desensitization in cancer patients.

P176 TYK2 inhibition attenuates the stimulation of sensory neurons with hyper-IL-6

Abstract Background Abdominal pain and diarrhoea are a cause of significant morbidity for people with colitis. Recent single cell RNA sequencing of sensory neurons innervating the gut highlights a likely role for sensory nerves in the pathophysiology of these symptoms based on the marked expression of cytokine receptors from the IL-6 receptor family in gut projecting sensory neurons. In keeping with the reported downstream signalling pathway for these receptors marked co-expression of TYK2 transcripts were also found along with IL-6 signal transducer (GP130) in colonic nociceptors characterised by expression of the capsaicin receptor TRPV1. These observations suggest that TYK2 inhibitors may ameliorate symptoms of pain and diarrhoea in people with colitis by preventing neurogenically mediated gut contractility and nociceptor activation. Methods TYK2 mediated sensory neuron activation: changes in the intracellular [Ca2+]i within thoracolumbar (T12-L5 spinal segments) dorsal root ganglia sensory neurons were examined in cells cultured overnight and loaded with Fluo-4-AM (30min) prior to imaging. Responses were measured to the hyper-IL-6 fusion protein of the soluble IL-6 receptor and IL-6, which transactivates the GP130 receptor subunit to mimic IL-6 family receptor signalling following pre-treatment with vehicle, the TYK2 inhibitor Deucravacitinib (100nM) or the pan-JAK inhibitor Pyridone-6 (2 μM). Afterwards cells were exposed to capsaicin (1 µM) followed by 50 mM KCl, to confirm the presence of nociceptors and neurons respectively by positive response. All experiments were performed using tissue from male C57/B6 mice euthanised by rising concentration of CO2followed by exsanguination in accordance with Schedule 1 of the UK Animals Scientific Procedures act (1986). Results Application of hyper-IL-6 produced a robust increase in [Ca2+]i levels in DRG sensory neurons which was significantly attenuated by pre-treatment with the TYK2 specific inhibitor Deucravacitinib (p<0.01) or the pan JAK inhibitor Pyridone-6 (p = <0.05). Conclusion Findings demonstrate that inhibition of TYK2 inhibitor prevents sensory neuron activation in response to IL-6 trans-signalling. This data suggests that TYK2 inhibitors may have utility for the treatment of abdominal pain in people with colitis.

Thermoring-based heat activation switches in the TRPV1 biothermometer

Non-covalent interactions in bio-macromolecules are individually weak but collectively important. How they take a concerted action in a complex biochemical reaction network to realize their thermal stability and activity is still challenging to study. Here graph theory was used to investigate how the temperature-dependent non-covalent interactions as identified in the 3D structures of the thermo-gated capsaicin receptor TRPV1 could form a systemic fluidic grid-like mesh network with topological grids constrained as the thermo-rings to govern heat-sensing. The results showed that the heat-evoked melting of the biggest grid initiated a matched temperature threshold to release the lipid from the active vanilloid site for channel activation. Meanwhile, smaller grids were required to stabilize heat efficacy. Altogether, the change in the total grid sizes upon the change in the total noncovalent interactions along the lipid-dependent gating pathway was necessary for the matched temperature sensitivity. Therefore, this grid thermodynamic model may be broadly significant for the structural thermostability and the functional thermoactivity of bio-macromolecules.

Members of the CUGBP Elav-like family of RNA-binding proteins are expressed in distinct populations of primary sensory neurons.

Primary sensory dorsal root ganglia (DRG) neurons are diverse, with distinct populations that respond to specific stimuli. Previously, we observed that functionally distinct populations of DRG neurons express mRNA transcript variants with different 3' untranslated regions (3'UTRs). 3'UTRs harbor binding sites for interaction with RNA-binding proteins (RBPs) for transporting mRNAs to subcellular domains, modulating transcript stability, and regulating the rate of translation. In the current study, analysis of publicly available single-cell RNA-sequencing data generated from adult mice revealed that 17 3'UTR-binding RBPs were enriched in specific populations of DRG neurons. This included four members of the CUG triplet repeat (CUGBP) Elav-like family (CELF): CELF2 and CELF4 were enriched in peptidergic, CELF6 in both peptidergic and nonpeptidergic, and CELF3 in tyrosine hydroxylase-expressing neurons. Immunofluorescence studies confirmed that 60% of CELF4+ neurons are small-diameter C fibers and 33% medium-diameter myelinated (likely Aδ) fibers and showed that CELF4 is distributed to peripheral termini. Coexpression analyses using transcriptomic data and immunofluorescence revealed that CELF4 is enriched in nociceptive neurons that express GFRA3, CGRP, and the capsaicin receptor TRPV1. Reanalysis of published transcriptomic data from macaque DRG revealed a highly similar distribution of CELF members, and reanalysis of single-nucleus RNA-sequencing data derived from mouse and rat DRG after sciatic injury revealed differential expression of CELFs in specific populations of sensory neurons. We propose that CELF RBPs may regulate the fate of mRNAs in populations of nociceptors, and may play a role in pain and/or neuronal regeneration following nerve injury.

Opening of capsaicin receptor TRPV1 is stabilized equally by its four subunits

Capsaicin receptor TRPV1 is a nociceptor for vanilloid molecules, such as capsaicin and resiniferatoxin (RTX). Even though cryo-EM structures of TRPV1 in complex with these molecules are available, how their binding energetically favors the open conformation is not known. Here, we report an approach to control the number of bound RTX molecules (0-4) in functional rat TRPV1. The approach allowed direct measurements of each of the intermediate open states under equilibrium conditions at both macroscopic and single-molecule levels. We found that RTX binding to each of the four subunits contributes virtually the same activation energy, which we estimated to be 1.70 to 1.86kcal/mol and found to arise predominately from destabilizing the closed conformation. We further showed that sequential bindings of RTX increase open probability without altering single-channel conductance, confirming that there is likely a single open-pore conformation for TRPV1 activated by RTX.

The Vanilloid (Capsaicin) Receptor TRPV1 in Blood Pressure Regulation: A Novel Therapeutic Target in Hypertension?

Today's sedentary lifestyle with excess food and little exercise increases the number of people with hypertension, a major risk factor for stroke. New knowledge of treatments in this field is of utmost importance. In animal experiments, the activation by capsaicin of TRPV1-expressing sensory afferents evokes a drop in blood pressure by triggering the Bezold-Jarisch reflex. In hypertensive rats, capsaicin reduces blood pressure. Conversely, genetic ablation of the TRPV1 receptor results in elevated nocturnal (but not diurnal) blood pressure. These observations imply a therapeutic potential for TRPV1 activation in hypertensive patients. Indeed, in a major epidemiological study involving 9273 volunteers, dietary capsaicin was found to lower the risk for hypertension. New research indicates that the mechanism of action of capsaicin on blood pressure regulation is far more complex than previously thought. In addition to the well-recognized role of capsaicin-sensitive afferents in blood pressure regulation, TRPV1 seems to be expressed both in endothelial cells and vascular smooth muscle. This review aims to evaluate the therapeutic potential of TRPV1-targeting drugs in hypertensive patients.

Drosophila pain sensitization and modulation unveiled by a novel pain model and analgesic drugs.

In mammals, pain is regulated by the combination of an ascending stimulating and descending inhibitory pain pathway. It remains an intriguing question whether such pain pathways are of ancient origin and conserved in invertebrates. Here we report a new Drosophila pain model and use it to elucidate the pain pathways present in flies. The model employs transgenic flies expressing the human capsaicin receptor TRPV1 in sensory nociceptor neurons, which innervate the whole fly body, including the mouth. Upon capsaicin sipping, the flies abruptly displayed pain-related behaviors such as running away, scurrying around, rubbing vigorously, and pulling at their mouth parts, suggesting that capsaicin stimulated nociceptors in the mouth via activating TRPV1. When reared on capsaicin-containing food, the animals died of starvation, demonstrating the degree of pain experienced. This death rate was reduced by treatment both with NSAIDs and gabapentin, analgesics that inhibit the sensitized ascending pain pathway, and with antidepressants, GABAergic agonists, and morphine, analgesics that strengthen the descending inhibitory pathway. Our results suggest Drosophila to possess intricate pain sensitization and modulation mechanisms similar to mammals, and we propose that this simple, non-invasive feeding assay has utility for high-throughput evaluation and screening of analgesic compounds.

Capsaicin and cancer: Guilty as charged or innocent until proven guilty?

ABSTRACT With an estimated 2 billion chili pepper connoisseurs worldwide, the human exposure to capsaicin is enormous. Therefore, the question whether nutritional capsaicin is a cancer causing or cancer preventive agent is of utmost importance. The gamut of human epidemiology studies suggests that capsaicin in modest, “restaurant-like” doses is not only safe to eat, but it may even provide health benefits, such as lower cancer-related death rate. Very “hot” food is, however, probably better avoided. Importantly, no increased cancer risk was reported in patients following topical (skin or intravesical) capsaicin therapy. Aberrant capsaicin receptor TRPV1 expression was noted in various cancers with potential implications for cancer therapy, diagnosis and prognostication. Indeed, capsaicin can kill cancer cells by a combination of on- and off-target mechanisms, though it remains unclear if this can be exploited for therapeutic purposes. The literature on capsaicin and cancer is vast and controversial. This review aims to find answers to questions that are relevant for our daily life and medical practice.

Dietary Capsaicin: A Spicy Way to Improve Cardio-Metabolic Health?

Today's sedentary lifestyle with too much food and too little exercise has made metabolic syndrome a pandemic. Metabolic syndrome is a major risk factor for type-2 diabetes and cardiovascular disease. New knowledge of medical and nutraceutical intervention in the early stages of metabolic syndrome is central to prevent these deadly complications. People who eat chili pepper on a regular basis seem to stay healthier and live longer than those who do not. Animal experiments suggest a therapeutic potential for dietary capsaicin, the active principle in hot chili pepper, to reduce the risk of developing metabolic syndrome. This is an attractive theory since capsaicin has been a culinary staple for thousands of years, and is generally deemed safe when consumed in hedonically acceptable doses. The broad expression of the capsaicin receptor TRPV1 in metabolically active tissues lends experimental support to this theory. This review critically evaluates the available experimental and clinical evidence for and against dietary capsaicin being an effective dietary means to improve cardio-metabolic health. It comes to the conclusion that although a chili pepper-rich diet is associated with a reduced risk of dying due to cardiovascular disease, dietary capsaicin has no clear effect on blood glucose or lipid profiles. Therefore, the reduced mortality risk may reflect the beneficial action of digested capsaicin on gut microbiota.

The industrial solvent 1,4-dioxane causes hyperalgesia by targeting capsaicin receptor TRPV1

BackgroundThe synthetic chemical 1,4-dioxane is used as industrial solvent, food, and care product additive. 1,4-Dioxane has been noted to influence the nervous system in long-term animal experiments and in humans, but the molecular mechanisms underlying its effects on animals were not previously known.ResultsHere, we report that 1,4-dioxane potentiates the capsaicin-sensitive transient receptor potential (TRP) channel TRPV1, thereby causing hyperalgesia in mouse model. This effect was abolished by CRISPR/Cas9-mediated genetic deletion of TRPV1 in sensory neurons, but enhanced under inflammatory conditions. 1,4-Dioxane lowered the temperature threshold for TRPV1 thermal activation and potentiated the channel sensitivity to agonistic stimuli. 1,3-dioxane and tetrahydrofuran which are structurally related to 1,4-dioxane also potentiated TRPV1 activation. The residue M572 in the S4-S5 linker region of TRPV1 was found to be crucial for direct activation of the channel by 1,4-dioxane and its analogs. A single residue mutation M572V abrogated the 1,4-dioxane-evoked currents while largely preserving the capsaicin responses. Our results further demonstrate that this residue exerts a gating effect through hydrophobic interactions and support the existence of discrete domains for multimodal gating of TRPV1 channel.ConclusionsOur results suggest TRPV1 is a co-receptor for 1,4-dioxane and that this accounts for its ability to dysregulate body nociceptive sensation.

Hypothermia promotes mitochondrial elongation In cardiac cells via inhibition of Drp1

Hypothermia is a valuable clinical tool in mitigating against the consequences of ischemia in surgery, stroke, cardiac arrest and organ preservation. Protection is afforded principally by a reduction of metabolism, manifesting as reduced rates of oxygen uptake, preservation of ATP levels, and a curtailing of ischemic calcium overload. The effects of non-ischemic hypothermic stress are relatively unknown. We sought to investigate the effects of clinically mild-to-severe hypothermia on mitochondrial morphology, oxygen consumption and protein expression in normoxic hearts and cardiac cells.Normoxic perfusion of rat hearts at 28–32 °C was associated with inhibition of mitochondrial fission, evidenced by a reduced abundance of the active phosphorylated form of the fission receptor Drp1 (pDrp1S616). Abundance of the same residue was reduced in H9c2 cells subjected to hypothermic culture (25–32 °C), in addition to a reduced abundance of the Drp1 receptor MFF. Hypothermia-treated H9c2 cardiomyocytes exhibited elongated mitochondria and depressed rates of mitochondrial-associated oxygen consumption, which persisted upon rewarming. Hypothermia also promoted a reduction in mRNA expression of the capsaicin receptor TRPV1 in H9c2 cells. When normothermic H9c2 cells were transfected with TRPV1 siRNA we observed reduced pDrp1S616 and MFF abundance, elongated mitochondria, and reduced rates of mitochondrial-associated oxygen consumption, mimicking the effects of hypothermic culture.In conclusion hypothermia promoted elongation of cardiac mitochondria via reduced pDrp1S616 abundance which was also associated with suppression of cellular oxygen consumption. Silencing of TRPV1 in H9c2 cardiomyocytes reproduced the morphological and respirometric phenotype of hypothermia. This report demonstrates a novel mechanism of cold-induced inhibition of mitochondrial fission.

Accelerated Production and Maturation of Human Induced Pluripotent Stem Cell Derived Nociceptors

The scarce availability and prohibitive costs of primary human DRG tissues limits the scalability of in vitro models for pain drug discovery and disease modeling. Human induced pluripotent stem cell (hiPSC)-derived sensory neurons provide a near-limitless amount of tissue for these applications. Using an accelerated, step-wise, directed differentiation protocol that proceeds through a novel ‘primal ectoderm’ population, we are able to manufacture nociceptors rapidly and at scale by relying on a simplified chemically-defined basal medium, recombinant growth factors and small molecules. Following production and only one week of maturation, these nociceptors have a consistent molecular phenotype and express key therapeutic pain targets such as human voltage-gated sodium channels Nav1.7, 1.8, and 1.9, which are primarily responsible for eliciting nociceptor action potentials, as well as the human capsaicin receptor TRPV1. Using whole-cell patch clamp electrophysiology, we demonstrate that these neurons fire action potentials and possess burst-firing ability by day 9 in vitro as assessed by performing stepwise depolarizing current injections in current clamp mode. We also show these neurons exhibit inward and outward currents in response to voltage steps in voltage clamp mode. We have also demonstrated that these neurons are responsive to ligands such as capsaicin using calcium imaging region-of-interest analysis. Greater than 30% of neuronal cell bodies respond to 100 nM capsaicin following an additional 2 to 3 weeks of maturation. Importantly, this response was ablated by capsazepine, thus confirming response specificity. These data demonstrate that hiPSC nociceptors have the potential to screen and identify novel analgesic or anti-nociceptive compounds for use in humans.

Gating of the capsaicin receptor TRPV1 by UVA-light and oxidants are mediated by distinct mechanisms

Redox-sensitivity is a common property of several transient receptor potential (TRP) ion channels. Oxidants and UVA-light activate TRPV2 by oxidizing methionine pore residues which are conserved in the capsaicin-receptor TRPV1. However, the redox-sensitivity of TRPV1 is regarded to depend on intracellular cysteine residues. In this study we examined if TRPV1 is gated by UVA-light, and if the conserved methionine residues are relevant for redox-sensitivity of TRPV1. Patch clamp recordings were performed to explore wildtype (WT) and mutants of human TRPV1 (hTRPV1).UVA-light induced hTRPV1-mediated membrane currents and potentiated both proton- and heat-evoked currents. The reducing agent dithiothreitol (DTT) prevented and partially reversed UVA-light induced sensitization of hTRPV1. UVA-light induced sensitization was reduced in the mutant hTRPV1-C158A/C387S/C767S (hTRPV1-3C). The remaining sensitivity to UVA-light of hTRRPV1-3C was not further reduced upon exchange of the methionine residues M568 and M645. While UVA-induced sensitization was reduced in the protein kinase C-insensitive mutant hTRPV1-S502A/S801A, the PKC-inhibitors chelerythrine chloride, staurosporine and Gö6976 did not reduce UVA-induced effects on hTRPV1-WT. While hTRPV1-3C was insensitive to the cysteine-selective oxidant diamide, it displayed a residual sensitivity to H2O2 and chloramine-T. However, the exchange of M568 and M645 in hTRPV1-3C did not further reduce these effects.Our data demonstrate that oxidants and UVA-light gate hTRPV1 by cysteine-dependent as well as cysteine-independent mechanisms. In contrast to TRPV2, the methionine residues 568 and 645 seem to be of limited relevance for redox-sensitivity of hTRPV1. Finally, UVA-light induced gating of hTRPV1 does not seem to require activation of protein kinase C.

The role of pruriceptors in enhancing sensitivity to pruritogens in a murine chronic compression model of dorsal root ganglion

Chronic pruritus is a symptom that commonly observed in neurological diseases. It has been hypothesized that the chronic pruritus may result from sensitization of itch-signaling pathways but the mechanisms remain obscure. In this study, we established a mouse model of chronic compression of dorsal root ganglion (CCD) and injected various pruritogenic and algogenic agents intradermally to the calf skin ipsilateral to the compressed dorsal root ganglion (DRG). Compared to the naïve mice, a significant increase in itch-related behaviors was observed in the CCD mice after the injection of pruritogens including histamine and BAM8-22, but not after the injection of capsaicin, although all the above agents evoked enhanced pain-related behaviors toward the injected site. In addition, we investigated if pruritogen-evoked activities of DRG neurons were enhanced in this model. In vivo calcium imaging revealed that compressed DRG neurons exhibited enhanced responses to histamine and BAM8-22. Immunoflorescent staining also showed that the histamine receptor H1 and the capsaicin receptor TRPV1 were significantly upregulated in DRG neurons. Our findings indicated that the sensitization of primary pruriceptive neurons may underlie the enhanced itch sensation after chronic compression of DRG in the mice, and may play a role in chronic pruritus in neurological diseases.

New capsaicin analogs as molecular rulers to define the permissive conformation of the mouse TRPV1 ligand-binding pocket.

The capsaicin receptor TRPV1 is an outstanding representative of ligand-gated ion channels in ligand selectivity and sensitivity. However, molecular interactions that stabilize the ligand-binding pocket in its permissive conformation, and how many permissive conformations the ligand-binding pocket may adopt, remain unclear. To answer these questions, we designed a pair of novel capsaicin analogs to increase or decrease the ligand size by about 1.5 Å without altering ligand chemistry. Together with capsaicin, these ligands form a set of molecular rulers for investigating ligand-induced conformational changes. Computational modeling and functional tests revealed that structurally these ligands alternate between drastically different binding poses but stabilize the ligand-binding pocket in nearly identical permissive conformations; functionally, they all yielded a stable open state despite varying potencies. Our study suggests the existence of an optimal ligand-binding pocket conformation for capsaicin-mediated TRPV1 activation gating, and reveals multiple ligand-channel interactions that stabilize this permissive conformation.

In Vivo Photopharmacology Enabled by Multifunctional Fibers.

Photoswitchable ligands can add an optical switch to a target receptor or signaling cascade and enable reversible control of neural circuits. The application of this approach, termed photopharmacology, to behavioral experiments has been impeded by a lack of integrated hardware capable of delivering both light and compounds to deep brain regions in moving subjects. Here, we devise a hybrid photochemical genetic approach to target neurons using a photoswitchable agonist of the capsaicin receptor TRPV1, red-AzCA-4. Using multifunctional fibers with optical and microfluidic capabilities, we delivered a transgene coding for TRPV1 into the ventral tegmental area (VTA). This sensitized excitatory VTA neurons to red-AzCA-4, allowing us to optically control conditioned place preference in mice, thus extending applications of photopharmacology to behavioral experiments. Applied to endogenous receptors, our approach may accelerate future studies of molecular mechanisms underlying animal behavior.

Vitamin D is an endogenous partial agonist of the transient receptor potential vanilloid 1 channel.

Key points 25‐Hydroxyvitamin D (25OHD) is a partial agonist of TRPV1 whereby 25OHD can weakly activate TRPV1 yet antagonize the stimulatory effects of the full TRPV1 agonists capsaicin and oleoyl dopamine.25OHD binds to TRPV1 within the same vanilloid binding pocket as capsaicin.25OHD inhibits the potentiating effects of PKC‐mediated TRPV1 activity.25OHD reduces T‐cell activation and trigeminal neuron calcium signalling mediated by TRPV1 activity.These results provide evidence that TRPV1 is a novel receptor for the biological actions of vitamin D in addition to the well‐documented effects of vitamin D upon the nuclear vitamin D receptor.The results may have important implications for our current understanding of certain diseases where TRPV1 and vitamin D deficiency have been implicated, such as chronic pain and autoimmune diseases, such as type 1 diabetes. The capsaicin receptor TRPV1 plays an important role in nociception, inflammation and immunity and its activity is regulated by exogenous and endogenous lipophilic ligands. As vitamin D is lipophilic and involved in similar biological processes as TRPV1, we hypothesized that it directly regulates TRPV1 activity and function. Our calcium imaging and electrophysiological data demonstrate that vitamin D (25‐hydroxyvitamin D (25OHD) and 1,25‐hydroxyvitamin D (1,25OHD)) can weakly activate TRPV1 at physiologically relevant concentrations (100 nM). Furthermore, both 25OHD and 1,25OHD can inhibit capsaicin‐induced TRPV1 activity (IC50 = 34.3 ± 0.2 and 11.5 ± 0.9 nM, respectively), but not pH‐induced TRPV1 activity, suggesting that vitamin D interacts with TRPV1 in the same region as the TRPV1 agonist capsaicin. This hypothesis is supported by our in silico TRPV1 structural modelling studies, which place 25OHD in the same binding region as capsaicin. 25OHD also attenuates PKC‐dependent TRPV1 potentiation via interactions with a known PKC phospho‐acceptor residue in TRPV1. To provide evidence for a physiological role for the interaction of vitamin D with TRPV1, we employed two different cellular models known to express TRPV1: mouse CD4+ T‐cells and trigeminal neurons. Our results indicate that 25OHD reduces TRPV1‐induced cytokine release from T‐cells and capsaicin‐induced calcium activity in trigeminal neurons. In summary, we provide evidence that vitamin D is a novel endogenous regulator of TRPV1 channel activity that may play an important physiological role in addition to its known effects through the canonical nuclear vitamin D receptor pathway.

The capsaicin receptor TRPV1 is the first line defense protecting from acute non damaging heat: a translational approach

BackgroundPain is the vital sense preventing tissue damage by harmful noxious stimuli. The capsaicin receptor TRPV1 is activated by noxious temperatures, however, acute heat pain is only marginally affected in mice after TRPV1 knockout but completely eliminated in mice lacking TRPV1 positive fibers. Exploring contribution of candidate signal transduction mechanisms to heat pain in humans needs translational models.MethodsWe used focused, non-damaging, short near-infrared laser heat stimuli (wavelength 1470/1475 nm) to study the involvement of TRPV1-expressing nerve fibers in the encoding of heat pain intensity. Human psychophysics (both sexes) were compared to calcium transients in native rat DRG neurons and heterologously expressing HEK293 cells.ResultsHeating of dermal and epidermal nerve fibers in humans with laser stimuli of ≥ 2.5 mJ (≥ 25 ms, 100 mW) induced pain that increased linearly as a function of stimulus intensity in double logarithmic space across two orders of magnitude and was completely abolished by desensitization using topical capsaicin. In DRG neurons and TRPV1-expressing HEK cells, heat sensitivity was restricted to capsaicin sensitive cells. Strength duration curves (2–10 ms range) and thresholds (DRGs 0.56 mJ, HEK cells 0.52 mJ) were nearly identical. Tachyphylaxis upon repetitive stimulation occurred in HEK cells (54%), DRGs (59%), and humans (25%).ConclusionTRPV1-expressing nociceptors encode transient non-damaging heat pain in humans, thermal gating of TRPV1 is similar in HEK cells and DRG neurons, and TRPV1 tachyphylaxis is an important modulator of heat pain sensitivity. These findings suggest that TRPV1 expressed in dermal and epidermal populations of nociceptors serves as first line defense against heat injury.